Looks like this would be a converted version:

public class Headbobber: MonoBehaviour 
{

 private float timer = 0.0f;
 float bobbingSpeed = 0.18f;
 float bobbingAmount = 0.2f;
 float midpoint = 2.0f;

 void Update () {
    float waveslice = 0.0f;
    float horizontal = Input.GetAxis("Horizontal");
    float vertical = Input.GetAxis("Vertical");

 Vector3 cSharpConversion = transform.localPosition; 

    if (Mathf.Abs(horizontal) == 0 && Mathf.Abs(vertical) == 0) {
       timer = 0.0f;
    }
    else {
       waveslice = Mathf.Sin(timer);
       timer = timer + bobbingSpeed;
       if (timer > Mathf.PI * 2) {
          timer = timer - (Mathf.PI * 2);
       }
    }
    if (waveslice != 0) {
       float translateChange = waveslice * bobbingAmount;
       float totalAxes = Mathf.Abs(horizontal) + Mathf.Abs(vertical);
       totalAxes = Mathf.Clamp (totalAxes, 0.0f, 1.0f);
       translateChange = totalAxes * translateChange;
       cSharpConversion.y = midpoint + translateChange;
    }
    else {
       cSharpConversion.y = midpoint;
    }

 transform.localPosition = cSharpConversion;
 }



}

It's hard to know because it seems undefined variables pop up out of no where. Unfortunately I don't have any projects to test this code for you, but the console doesn't give any compile errors for me. Also, the cSharpConversion V3 is there because c# doesn't like transform.localPosition.y = midpoint; :(

Happy coding.